1. Field of the Invention
The present invention concerns the field of guided wave optical devices including a grating, that is to say including a part through which an optical wave travels and in which an optical parameter varies in an alternating manner along the path of the light.
2. Description of the Related Art
The invention concerns codirectional couplers and couplers, guided wave Bragg reflectors, fibre type mode converters or Bragg reflectors for VCSEL cavities, for example.
The above devices filter optical waves. They eliminate waves with wavelengths outside a chosen range.
Generally speaking, an optical filter is of good quality when it selects wavelengths to be transmitted and wavelengths to be rejected precisely. In other words, the filter is of good quality if its spectral response takes substantially constant values close to 1 in the range of wavelengths to be transmitted and substantially constant values close to 0 outside of that range.
The spectral responses of current grating filters feature secondary lobes, however.
Various methods have been proposed for producing optical filters in which such secondary lobes are small in amplitude, in other words filters with a high rejection rate. The skilled person knows such methods as apodisation methods.
When the grating device is the site of interference formally described using the principle of coupled modes, apodisation methods have been proposed that consist in modulating an amplitude of variation of the coupling coefficient along the path of the light.
A first such method of varying the amplitude of the coupling coefficient which is routinely used in designing codirectional and contradirectional couplers is implemented by varying an interguide distance. This method is described in B. E. Little, C. Wu, W. P. Huang, xe2x80x9cSynthesis of ideal window filter response in grating-assisted couplersxe2x80x9d, Optics Lett., vol. 21, pp. 725-727, 1996; B. E. Little, C. Wu, W. P. Huang, xe2x80x9cSynthesis of codirectional couplers with ultralow side lobes and minimum bandwidthxe2x80x9d, Optics Lett., vol. 22, pp. 1259-1261, 1995; and G. H. Song, xe2x80x9cToward the ideal codirectional Bragg filter with an acousto-optic-filter designxe2x80x9d, J. Lightwave Technol., vol. 13, pp. 470-480, 1995. It produces in the order of 30 dB to 40 dB apodisation of the secondary lobes.
The accompanying FIG. 1 is a top view of an interguide distance variation codirectional coupler of the above kind.
In this device the distance between two guides is caused to oscillate by transverse undulation of one of the two guides which yields an oscillatory distribution of the coupling coefficient along the undulating guide, in accordance with the coupled mode principle. The period of the oscillation determines the initial wavelength or the phase lock length selected by the device.
The undulating guide also has a generally arcuate shape so that the interguide distance is minimum at the centre of the device and maximum at its ends. The average coupling coefficient calculated at each undulation therefore has a maximum in the central part and decreases progressively towards the ends of the guides.
This bell-shaped variation of the average coupling coefficient calculated at each undulation reduces the secondary lobes of the spectral response of the filter.
However, the above method has a major drawback associated with the fact that the average interguide distance must undulate with an amplitude in the range of 2 xcexcm to 5 xcexcm, the amplitude of sinusoidal variation being in the order of 1 xcexcm. Implementing it therefore requires great precision in the variations of the interguide distance, which is difficult to achieve in practice.
Moreover, the coupling coefficient is related to the interguide distance by a non-linear function and this tends to accentuate the negative effects of errors and uncertainties on the interguide distance.
The method is also difficult to implement for vertical couplers and cannot be applied to components in which there is only one guide, for example Bragg reflectors like that shown in the accompanying FIG. 3 or light mode converters.
A second method, known as the modulation amplitude variation method, consists in varying the coupling coefficient by varying a corrugation profile or varying the refractive index in a fibre.
When the corrugation profile is varied, it is in practice difficult to control corrugation amplitudes so that the modulation of the coupling coefficient is sufficiently precise. The corrugation amplitudes required are generally below 1 xcexcm.
When the index in the fibre is varied, very complex and very exacting ultraviolet exposure techniques are employed. These methods are described in J. Albert, K. O. Hill, D. C. Jonson, F. Bilodeau, M. J. Rooks, xe2x80x9cMoire phase masks for automatic pure apodisation of fibre Bragg gratingsxe2x80x9d, Electronic Lett., vol. 32 pp. 2260-2261, 1996; P. Kashyap, A. Swanton, D. J. Armes, xe2x80x9cSimple technique for apodising chirped and unchirped fibre gratingsxe2x80x9d, Electronic Lett., vol. 32, pp. 1226-1227, 1996; J. Albert, K. O. Hill, B. Malo, S. Thxc3xa9riault, F. Bilodeau, D. C. Jonson, L. E. Erickson, xe2x80x9cApodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiencyxe2x80x9d, Electronics Lett., vol. 31, pp. 222-223, 1995; and B. Malo, S. Thxc3xa9riault, D. C. Jonson, F. Bilodeau, J. Albert, K. O. Hill, xe2x80x9cApodised in fibre Bragg grating reflectors photoimprinted using a phase maskxe2x80x9d, Electronic Lett., vol. 31, pp. 223-225, 1995 and are specifically addressed to gratings inscribed in the fibres.
A third method, known as the cyclic ration variation method, is described in H. Sakata, xe2x80x9cSide lobe suppression in grating-assisted wavelength-selective couplersxe2x80x9d, Optics Lett., vol. 17, pp. 463-465, 1992. The cyclic ratio is defined over a period of the grating as the ratio between the length of the part of the period in which the coupling coefficient is positive and the length of the part of the period in which the coupling coefficient is negative. This method varies this ratio along the path of the light.
FIG. 7 shows a grating optical filter of a type known per se in which the cyclic ratio is modified along the length of the filter.
The filter comprises a central guide flanked by portions adapted to modify the value of the coupling coefficient in the parts of the central guide level with these portions.
To be more precise, the portions of the central guide that are flanked on the right have a negative coupling coefficient and the portions of the central guide that are flanked on the left have a positive coupling coefficient.
The central guide can be considered as a succession of sections each made up of a guide part in which the coupling coefficient is negative followed by a guide part in which the coupling coefficient is positive.
In FIGS. 7 to 9 dashed lines have been drawn between the successive sections constituting the filter and the successive sections have been numbered from 1 to 8.
These sections are all the same length. In other words, the lateral portions are disposed so that each pair consisting of a righthand portion and a lefthand portion has a constant length along the guide.
FIG. 9 shows the distribution of the coupling coefficient along the filter in the direction of increasing section numbers from section 4 to section 8.
In the graph shown in FIG. 9 the abscissa axis therefore plots a distance z measured along the filter in the direction of increasing section numbers and the ordinate axis plots the value of the coupling coefficient k at the point of the guide concerned.
A section of the filter therefore consists of a succession of two sub-sections, one in which the coupling coefficient is positive and the other in which it is negative, the absolute amplitudes being substantially equal.
The sections therefore form lobes, each having the same amplitude, which is constant along the filter.
According to the cyclic ratio variation principle that is known per se, the ratio between the length of the negative lobe and the length of the positive lobe within each section is not the same for all sections of the grating.
In other words, the central sections are each divided into two substantially equal halves, one in which the coupling coefficient is negative and the other in which the coupling coefficient is positive, and the sections at the ends of the filter have a great difference in length between their negative coupling coefficient part and their positive coupling coefficient part.
As shown in FIG. 7, the section has a positive coupling coefficient part that is twice as long as its negative coupling coefficient part.
This disproportion between the positive part and the negative part of each section increases in the direction away from the centre of the device. In other words, the guide therefore has a ratio between the length of the righthand portion and the length of the lefthand portion of a pair which is maximum at the centre of the guide and progressively decreases in the direction towards the ends of the guide.
The sections therefore retain a constant length along the filter, the length of the negative coupling coefficient part progressively decreases in the direction away from the centre of the device and the length of the positive coupling coefficient part, complementary to the negative part, progressively increases in the direction away from the centre of the device.
An average coupling coefficient Km(i) defined as the average of the coupling coefficient over a section with index i has a distribution along the guide as shown in FIG. 8.
Conforming to the distribution of the cyclic ratio, the coupling coefficient Km(i) calculated in each section of index i has low values at the ends and a maximum at the centre of the guide.
This kind of distribution of the average coupling coefficient in each section is known to yield an apodised spectral response.
This method uses a constant grating height and a constant interguide distance.
This method requires control of the length of the sections that is easier to achieve than the geometrical control required in the methods previously referred to. This applies in particular to codirectional couplers, in which the grating has a pitch that is generally several tens of microns.
This method nevertheless has a major drawback, in that the devices obtained frequently cause high radiation losses.
Moreover, in devices obtained by this method, the phase lock length changes with the value of the cyclic ratio, which causes additional problems in designing the filter. These problems are discussed in H. Sakata, xe2x80x9cSide lobe suppression in grating-assisted wavelength-selective couplersxe2x80x9d, Optics Lett., vol. 17, pp. 463-465, 1992.
Other apodisation methods similar to the cyclic ratio variation method have also been proposed. These methods are discussed in E. Shibata, S. Oku, Y. Kondo, T. Tamamura, M. Naganuma, xe2x80x9cSemiconductor monolithic wavelength selective router using a grating switch integrated with a directional couplerxe2x80x9d, J. Lightwave Technol., vol. 14, pp. 1027-1032, 1996; Q. Guo, W. P. Huang, xe2x80x9cPolarisation-independent optical filters based on co-directional phase-shifted grating assisted couplers: theory and decisionxe2x80x9d, IEEE Proc.-Optoelectron., vol. 143, pp. 173-177, 1996; and Y. Shibata, T. Tamamura, S. Oku, Y. Kondo, xe2x80x9cCoupling coefficient modulation of waveguide grating using sampled gratingxe2x80x9d, IEEE Photonics Technol. Lett., vol. 6, pp. 1222-1224, 1994.
In one of these methods the filter is made up of parts with gratings and parts without gratings. Apodisation is effected by varying the ratio between the lengths of the parts with gratings and the lengths of the parts without gratings along the filter. FIG. 4 is a diagrammatic representation of a filter obtained by this method.
These filters have many sections with no grating and therefore have the disadvantage of a particularly great overall length.
Also, this method would not seem to be able to achieve apodisation levels of the secondary lobes greater than 20 dB.
Also known per se are methods of modifying the spectral response in which a grating is made whose period length, also known as the pitch, varies along the grating. These so-called xe2x80x9cchirpxe2x80x9d methods are routinely used in the manufacture of Bragg reflectors.
R. Kashyap, xe2x80x9cDesign of step chirped fibre gratingsxe2x80x9d, Optics Commun., vol. 136, pp. 461-469, 1997; and D. xc3x96stling, H. E. Engan, xe2x80x9cBroadband spatial mode conversion by chirped fibre bendingxe2x80x9d, Optics Lett., vol. 1, pp. 192-194, 1996 propose such methods in which a linear or quasi-linear increasing monotonous variation of the pitch of the grating is used, with the aim of widening the spectral response of the filter. This method does not reduce the level of the secondary lobes.
FIGS. 5 and 6 show devices obtained by these methods.
In these methods, the apodisation of the secondary lobes is effected by conventional methods of varying the average coupling coefficient per section, as discussed in J. Marti, D. Pastor, M. Tortola, J. Capmany, A. Montera, xe2x80x9cOn the use of tapered linearly chirped gratings as dispersion-induced equalizers in SCM systemsxe2x80x9d, J. Lightwave Technol., vol. 15, pp. 179-187, 1997.
The main aim of the present invention is to propose a grating-type optical filter device that is apodised by a method that is not subject to the drawbacks previously discussed.
This aim is achieved in accordance with the invention by a device adapted to have an optical wave travel through it and to filter that wave in terms of wavelength, in which device an optical parameter of the device varies along the path of the wave in such a manner that the device has a series of sections each formed of two successive segments, one in which the values of the optical parameter are less that an average value and the other in which the values of the optical parameter are greater than the average value, characterized in that the device has at least one zone in which the sections have lengths alternately less than and greater than an average length of the sections in that zone.